I have a 740iL with basically the same engine as the 4.4 X5. It is a fantastic engine.
I have no doubt that the 4.4 litre will be an excellent choice - BUT....
We live in a world where practicality is often a concern. Doubtless, if you have the money to buy one of these machines new (as it seems), the petrol consumption isn't an issue. Just remember that the X5 is a HUGE vehicle so even the 3 litre's petrol consumption might be higher than my 4 litre's because of the drag and weight factors.
The consumption might not be an issue, but the RANGE might be. Suppose you don't mind feeding the very thirsty 4.4litre V8, you will have to fill up often. The 3 litre will certainly offer far better consumption and a far more practical range.
If I were to buy an X5, I would go for the 3 litre DIESEL! The diesel has plenty power and LOADS of torque. Torque is stuff that lets you tow a boat up a mountain at 80mph. Also, the diesel is very fuel efficient and you won't have to fill up as often.
The choice in the end, is up to you. There's nothing as wonderful as hearing and feeling that 4.4litre V8 work. That in itself is worth the buy. But you asked which one and in my little world, the 3litre DIESEL would be the one that gets my signature